In the oilfield, it is often desirable to drill a new borehole in close proximity to an existing well. For example, oil well blowouts can often be “killed” by intersecting the blowing well with a new borehole containing a heavy fluid. The heavy fluid flows from the new well into the existing well bore to suppress any upward flows, thereby halting the blowout and making it possible to cap the original well head. In another example, certain oilfield production techniques (e.g., steam-assisted gravity drainage) employ generally parallel boreholes that are closely spaced together. To generate this configuration, an existing well is used as a reference to guide the drilling of the new, parallel borehole. In yet another example, offshore drillers need to drill many long-reach wells from a single platform. The steering techniques employed often require surprisingly complex paths to direct the wells to the desired formations without violating bending limits of the drill string. Drillers must carefully steer new boreholes clear of the existing wells.
Various techniques have been developed for steering new boreholes relative to existing wells. The earliest approach used for this type of problem employs two separate tools in the existing well and new borehole. See, e.g., U.S. Pat. Nos. 3,406,766 (“Method and devices for interconnecting subterranean boreholes” to H. J. Keller); 4,593,770 (“Method for preventing the drilling of a new well into one of a plurality of production wells” to G. L. Hoehn Jr.); 5,343,152 (“Electromagnetic homing system using MWD and current . . . injected at a target well” to A. F. Kuckes); and 5,676,212 (“Downhole electrode for well guidance system” to A. F. Kuckes). Although this approach has been successful and used widely, it necessitates a stop in the production, so that a wireline tool can be lowered to the existing well.
Another type of approach that eliminates that requirement is to use magnetization of the target well casing such as described in U.S. Pat. Nos. 4,072,200 (“Surveying of subterranean magnetic bodies from an adjacent off-vertical borehole” to F. J. Morris et al.); 4,458,767 (“Method for directionally drilling a first well to intersect a second well” to G. L. Hoehn Jr.); and 6,985,814 (“Well twinning techniques in borehole surveying” to G. McElhinney). Since the source is a passive, this latter approach eliminates the requirement for a transmitter. However very strong magnetization levels at the well casing must be maintained to utilize this kind of an approach, often making it infeasible.
Yet another approach is to stop the drilling and lower a wireline tool to sense the casing remotely as in U.S. Pat. No. 4,791,373 (“Subterranean target location by measurement of time-varying magnetic field vector in borehole” to A. F. Kuckes). Although this approach is being successfully employed in the industry, it requires the drilling equipment to be pulled up, the wireline tool run down, and the drilling tool deployed back down, resulting in an undesirable loss of time. U.S. Pat. No. 4,443,762 (“Method and apparatus for detecting the direction and distance to a target well casing” to A. F. Kuckes) describes a logging while drilling tool for detecting well casings, but its construction sharply limits the range and ability to detect a wide range of orientations for the existing well.
In a nutshell, existing techniques have been found to have very limited range or to require multiple logging runs (either sequential runs in the new borehole or a run in the existing well in cooperation with a run in the new borehole). Such operations in the existing well are generally undesirable or in some cases impossible (e.g., in a blowout). Sequential runs in the new borehole undesirably disrupt and delay the drilling process. As such, existing techniques are believed to be inadequate.